$(".signin").click(function() {
    this.className = "signin focus";
    $(".signup")[0].className = "signup";
    $(".input_signin")[0].className = "input_signin active";
    $(".input_signup")[0].className = "input_signup";
});
$(".signup").click(function() {
    this.className = "signup focus";
    $(".signin")[0].className = "signin";
    $(".input_signup")[0].className = "input_signup active";
    $(".input_signin")[0].className = "input_signin";
});